Electrode for discharge lamps



April 5, 1938. J. w. MARDEN ET AL ELECTRODE FOR DISCHARGE LAMPS FiledJune 30, 1954 INVENTOR J 14 MflFDE/V 6- MEI-572" ATTORNEY Z PatentedApr. 5, 1938 UNITED STATES PATENT OFFICE ELECTRODE FOR DISCHARGE LAMPSfacturing Company, E corporation of Pennsyl ast Pittsburgh, Pa., a

vania Application June 30, 1934, Serial No. 733,168

16 Claims. -(Cl. 176-126) This invention relates to discharge devicesand more particularly to an improved form of electrode therefor.

Although an electrode constructed in accordance with the presentinvention may find use in various forms of electrical devices it isespecially advantageous in connection with discharge lamps of the typeemploying a metallic vapor. A lamp of this character may include aninner and an outer bulb. The inner bulb contains the electrodes and oneor more rare gases as well as asmall quantity of vaporizable metal suchas mercury, sodium, cadmium, or mixtures of the same. The outer bulb orenvelope may be exhausted, or gas or air filled to provide a heatinsulative enclosure for the inner bulb or discharge chamber.

Heretofore electrodes used in the above devices have been made of coresof alkaline earth oxides, hydroxides, or silicates, with or withoutmetal mixtures, supported in spirals of tungsten wire. The oxides wereactivated by heating the tungsten coil to a sufiiciently hightemperature by the passage of electrical energy therethrough. Two leadwires were employed for this purpose. After activation, however, theleads were connected to make a single lead. Other electrodeconstructions have been used-such as a mesh disk of nickel or othermetal coated with an oxide to serve as an electron emission source, inwhich case only a single lead was employed and the oxides were activatedby high frequency induction heating.

In both types of electrodes the decomposition of the oxides resulted ina deposit of emission material over the entire effective surface of theelectrode which surface served to emit electrons and maintain adischarge during operation.

When electrical energy is applied for the operation of the above device,the initial discharge takes place by reason of the initial ionization ofthe rare gases and by reason of the electron emission of the material atrelatively low temperature and voltage. As the bulb heats up the mercuryis volatilized and its pressure rises with the result that the dischargecontracts and takes the form of an attenuated column between the twoelectrodes.

It has been found that devices operating with electrodes contructed asabove described are subject to certain disadvantages as for example theoxides which, during operation, must withstand high temperature andionic bombardment, vaporire and sputter, thus shortening the life of thedevice and causing blackening of the bulb, or the high temperaturecauses the nickel to melt with detrimental results. v

It is an object of the present invention to provide an electrode surfacecapable of supplying a copious flow of electrons to provide thenecessary 5 low voltage drop for ionizing the gas for starting and forpreventing destruction of the electrode surface during operation.

Another object of the invention is to provide an electrode having a lowtemperature electron 1 source and a high temperature electron source.

A further object of the invention is to provide a compound electrode fora discharge lamp, said electrode having one portion of low electrodedrop and another portion electron emissive at high temperature.

A still further object of the invention is to provide an electrodecomposed of an electron emissive oxide and an electron emissive metal.

Another object of the invention is to provide a 20 discharge device withan electrode which is elec tron emissive at relatively low temperatureand in preventing said electrode from being heated to high temperatureduring the operation of the device.

A further object of the invention is to provide a discharge device witha compound electrode consisting of a body which is electron emissive atrelatively low temperature and a second electrode arranged to protectsaid first electrode against heating to a relatively high temperatureduring operation of the device.

Other objects and advantages of the invention will be more clearlyunderstood from the following description together with the accompanyingdrawing in which Fig. 1 is a side elevational view of a discharge devicehaving electrodes constructed in accordance with the present invention;

Fig. 2 is an enlarged sectional view taken on 4 line 11-11 inFig. 1;

Fig. 3 is an enlarged side view of one of the. electrodes shown in Fig.1;

Fig. 4 is a perspective view of a modified form of the electrode;

Fig. 5 is a perspective view of another modified form of the electrodeand Fig. 6 shows the electrode made in the form of a plate with raisedportions extending above the thermionically active material.

The outer chamber may be provided with a reentrant stem l4 havingleading-in conductors l5 and I6 sealed in the press I! thereof. Anexhaust tube I8 may be provided for evacuating the chamber In, ifdesired. The inner chamber ll may be provided with the electrodes l2 andit of similar construction and a filling of any suitable ionizablemedium. The inner chamber may be supported in the outer chamber by meansof spring strips ll extending from a metallic collar Ii clamped ortightly fitting the tubular portion I! of the stem.

A conductive support rod 28 sealed through a wall at one end of thechamber II is electrically connected to lead wire It. The electrode i2is mounted on rod 23 at the end of the chamber and a connector piece 24electrically connects with a strip ii, the strips being electricallyconnected by the collar II. The electrode II is mounted at one end of asupport rod 2| sealed to the wall at the other end of the chamber II.The other end of the support rod II is electrically connected with aconductor 28 which is electrically connected with the lead wire I.

The spring strips II, which serve to hold the chamber II in spacedrelation to the wall of the outer chamber I. also serve to provide ashort gap between the electrodes for starting.

A lamp constructed for operation with electrodes made in accordance withthe present invention and operable on 110-volt circuit may have theelectrodes spaced about 12.5 centimeters apart; that is, to give adischarge length of about 12.5 centimeters. The diameter of thedischarge chamber may be 4.3! centimeters and a suitable impedance maybe employed to give an operating voltage of 63.5 volts at 6.35 amperes.The chamber may have a filling of argon of about 5 millimeters pressurefor low starting and mercury vapor at a pressure of about 200millimeters or more.

The gaseous filling for the discharge chamber may be mercury asmentioned but other metallic vapors such ascadmium, sodium, or acombination of such vapors may be employed. The rare gas may not alwaysbe necessary but it has been found advantageous in starting at lowvoltage.

With a lamp of the above type it is important to have electrodes whichwill provide a copious supply of electrons for initiating a discharge.It is, therefore, advantageous to provide electrode surfaces of electronactive materials such as barium oxide, strontium oxide or other alkalineearth compounds. These compounds may be decomposed to the oxides andactivated by high frequency heating in a manner well known in this art.

An electrode constructed in accordance with the present invention may beconsidered as a dual or compound electrode and may consist of a plate ordisk 21 and a member 20. The disk 21 serves as the supporting body formaterial capable of giving copious electron emission at relatively lowtemperature and in practice foraminous disks have been used consistingof nickel wire mesh which serves to hold the electron emissive material.

The material may be applied by spraying with the apertures orinterstices arefilled or the disk may be dipped in a bath to eflect therequired deposit of electron emissive material. After the disk or othersupporting body has been given a deposit of electron material, thematerial, in

accordance with the present invention. is removed from the surface ofthe body so that the portions of the mesh material around the aperturesare free of said material. It is to be understood that although goodresults have been obtained by using a metallic mesh to hold the electronemission material, such material may be in the form of a solid body as adisk or rod. When the electron emissive material is in such form, thehigh temperature electron emissive body may be secured in spacedrelation to the material, care said member it is soon dissipated whensaid member reaches the temperature at which it emits electrons andcarries the discharge.

It has been found that when constructing an electrode as for a lamp madein accordance with the above description, which operates at about 400watts, the bridge piece may be made of 16 mil tungsten wire and the diskmay be made of about 60 mesh nickel about one-half inch in diameter.Good results have been had when using four layers of mesh to provideapertures or interstices to receive and hold the electron emissivematerial. Although tungsten wire is satisfactory it is to be understoodthat other metals or refractory materials may be used for examplethorium, vanadium or molybdenum. The bridge piece may be varied in itsdimensions in accordance with the character of material employed.

Heretofore in devices of the character to which the present inventionrelates, the electrode, either of a wire woven or wound construction,was eniirely covered with the electron emissive material and served tosupport a discharge during the operation of the device.

In devices of the high intensity metal vapor type the discharge, asabove mentioned, instead of being widely diffused throughout the bulb,contracts into what is termed a pencil" form of discharge of highcurrent density. The coated surfaces of the electrodes are subjected toionic bombardment to the extent that they become heated to anexcessively high temperature and the nature of the coating is such thatit vaporizes or breaks away and spots or sputters with the result thatthe portions of the walls adjacent to the electrodes become blackened.Therefore, in addition to the disintegration of the electrodes whichshortens the life of the device, the light output is reduced by anobstructing light absorbing coatins.

It is the purpose of the present invention to provide avoltage-activated electrode which will operate to give a low electrodedrop and at the same time remove the detrimental'efl'ects above pointedout. This may be accomplished by diverting the discharge away from thecoated electrode surface after it has served its purpose in initiating adischarge. The present electrode is,

therefore, provided with means capable of supporting the discharge tothe substantial exclusion of the highly emitting material. It has beenfound possible to accomplish this result by constructing what may betermed a compound or dual function electrode. I

The disk 21 may be permitted to operate in the usual manner as astarting electrode. 0n the discharge side of the disk, however, ismounted the member 28 preferably of tungsten or other metal whichbecomeseflectively electron emissive at high temperatures and serves asan operating electrode. When the discharge starts the member 28 orsecondary electrode is heated by bombardment. It has been found inpractice when operating a lamp, as above described, that the said memberreaches a temperature of about 2500 degrees centigrade, at which it issufficiently electron emissive In the electrode construction shown inFigs. 1 to 3 the member 28 is provided with connector arms 3! integralwith the said member and electrically connected to the support rod 23.Thus when the' discharge is in full operation, the current betweenmembers 28 on opposite electrodes. is bypassed withrespect to the coateddisks 21 and the disks are therefore, not subjected to any actiondetrimental to their useful life. In fact the coating on the disk iskept substantially \intact ready to perform its starting function whennecessary.

It is obvious that the present electrode may be made in various formsand the dimensions of its component parts varied since the invention inits broad aspects is to provide a starting electrode and an operatingelectrode; the starting electrode being emissive at low temperature andprotected by the operating electrode which latter is electron emissiveat a relatively high temperaelectrode .in which a. coated mesh disk 32is mounted on a support disk -33 one end 34 extending through the rodand serving as the thermionically active secondary or operatingelectrode. The end 34 may be of the same material as the rod 33, as forexample, nickel in certain types of lamps or'it may consist of a tip oftungsten or other material of high melting point which is suflicientlyelectron emissive at the opcrating temperature of the device.

If desired, the electrode may be constructed, as shown in Figs. and 6.In Fig. 5 a bridge member 35 is provided with a plurality of turns 36 togive a plurality of points to receive the discharge. In Fig. 6 a disk 31is made from sheet metal with projections 38 and so formed as to providecavities, 39. for the deposit of the electron emissive material 40. Theprojections may be free of such material and serve as the operatingelectrode to carry the discharge when the device is in full operation.

The member 28 is so arranged with respectto the electron emissivematerial of the disk that the discharge during operation is spaced fromthe said material. For the purpose of centering the discharge column inthe envelope, the member 28 is formed as above described, with a pointor relatively small efiective area disposed on the longitudinal axis ofthe envelope. With this construction the discharge does not shift fromside to side as would occur if the member were divided as for example ifone member projected from each side of the disk. As shown in Figs. 5 and6, the discharge is carried by a plurality of points and although thedischarge may move from one point to the other or be divided between allthe points, it is held spaced from the electron emissive material andthe possibility of the discharge current taking a path through thedischarge material is remote.

It has been found that, when operating a discharge lamp provided withelectrodes constructed in accordance with the present invention,

during the operation of the device the body of 4 electron emissivematerial remains comparatively cool and inactive,'whi le the metallicmember which becomes electron emissive and sup,- ports the dischargereaches the high temperature mentioned above. In devices as heretoforeconstructed, the electron emissive material and its supportingstructureconstituting an electrode operated-at such a high temperaturethat vaporization was comparatively rapid and since such electrode wasrequired to give electron emission at low temperature for starting andto also operate at an excessively high temperatureduring operation, theuseful life of the device was short compared to the present device inwhich the starting electrode is protected and only functions for itsintended purpose.

It will be'evident that a device provided with electrodes constructed inaccordance with the present invention will not only operate at therequired starting potential but that the starting means will beconserved. The device, therefore, which is provided with a more durableand lastingelectrode for actual operation will give longor life and byreason of the removal of theabove mentioned defects, will be morepractical and eflicient.

Although a preferred embodiment of the invention is shown and describedherein, it is to be understood that modifications may be made thereinwithout departing from the spirit and scope of the invention as setforth in the appended claims.

What is claimed is:

1. A discharge device comprising a sealed envelope, a conductor sealedthrough a wall of said envelope, a plate mounted at one end of saidconductor and within said envelope, a deposit of electron emissivematerial on portions of said plate, other portions of said plate beingwithout a deposit of said material, and a member electron emissive atrelatively high temperature extending from said plate and disposed inthe discharge path.

2. A discharge device comprising a sealed en velope, a conductor sealedthrough a wall of said envelope, a plate mounted at one end of saidconductor and within said envelope, a deposit of electron emissivematerial on portions of said plate, other portions of said plate beingwithout a deposit of said material, a member extending across said plateand spaced therefrom, and arms electrically connecting said member tosaid conductor.

3. An electrode for a discharge device comprising a mesh disk, aconductive lead wire extending.

from one side of said disk, a coating of electron emissive material-onsaid disk, and a metallic bridge member extending across the other sideof said disk.

4. An electrode for a discharge device comprisjing a mesh d sk, aconductive support member extending from'one side of said disk, adeposit of electron emissive material disposed between the wires of saidmesh and a metallic bridge member on the other side of said disk andextending outwardly from said emissive material.

6. A voltage-activated electrode fora discharge device comprising a bodyhaving apertures to receive electron emissive material, the surroundingedges adjacent to the perimeters of said apertures being free from saidmaterial, and a metallic member electron emissive at high temperatureextending above the surface of said material and disposed in the path ofdischarge of said electron emissive material. I

7. A voltage-activated electrode for a discharge device comprising abody having apertures to receive' electron emissive material, thesurrounding edges adjacent to the perimeters of said apertures beingfree from said material, and a metallic member spaced from said body anddisposed in the discharge path to receive the discharge and serve as anoperating electrode, said member being electrically connected with saidbody.

8. A voltage-activated electrode for a discharge device comprising ametallic member, a conductive lead wire extending from one side of saidmember, a deposit of electron emissive material on said member, anothermember on the other side of said metallic member electrically connectedwith said lead wire and free from said emissive material, said lastmentioned member being positioned to serve as an operating electrode tothe substantial exclusion of the member having the deposit of electronemissive material..

9. Adischarge device comprising a sealed envelope, a mesh disk supportfor electron emission material, a conductor member sealed through thewall of said envelope and connected to one side of said support, adeposit of electron emissive material on portions of the other side ofsaid support,

other portions of said support being free of said electron emissivematerial, and a metallic member extending from said other side of saidsupport and electrically connected with said conduc-- material onportions of the surface of the other side of said support, otherportions of said support being free of said electron emissive material,and a metallic member spaced from said surface and connected with saidconductor member, in order to serve as an operating electrode taking themain discharge.

11. A discharge device comprising a sealed enveiope, a conductor sealedthrough a wall of said envelope, a plate having one side thereof securedto one end of said conductor and within said envelope, a deposit ofelectron emissive material on portions of the other side of said plate,other portions of said plate being without a deposit of said material, ametallic member extending from said other side of said plate, and meansfor electrically connecting said member with said conductor.

12. A discharge device comprising a sealed envelope, a conductor sealedthrough a wall of said envelope, a voltage-activated starting electrodehaving one side thereof secured to one end of said conductor and withinsaid envelope, a deposit of electron emissive material onsaid electrode,an operating electrode extending above said material and across saidstarting electrode and spaced aliases therefrom, and means forelectrically connecting said operating electrode with said conductor.

13. A ,high intensity vapor lamp comprising a sealed envelope, anionizable medium in said-envelope, a conductor sealed through the wallof. said envelope, a cold duplex electrode connected to said conductor,said electrode including a mesh support holding material electronemissive at a relatively low temperature and a given voltage,

sealed bulb, an ionizable vapor within said bulb.

' a conductor sealed through the wall at each end of said bulb, a coldelectrode connected with each conductor, each of said electrodes havinga mesh portion coated with an electron emissive material to ionize saidvapor when said electrodes are subjected to a difierence of potential,each of said electrodes having an extended portion positioned in thedischarge path, said extended portions being electron emissive whenheated by the discharge and so positioned with respect to said coatedportions that the discharge continues between said extended portionssubs'tantially to the exclusion of said coated portions.

15. A high intensity vapor lamp comprising a sealed tubular bulb, anionizable metallic vapor within said bulb, a small quantity of morereadily ionizable gas in said bulb. a conductor sealed through the wallat each end of said bulb, a cold electrode connected with each conductorand disposed in close proximity to the ends of said bulb, each of saidelectrodes having a mesh portion coated with a material electronemissive at a given voltage when cold to ionize said gas when saidelectrodes are subjected to a difference of potential, each of saidelectrodes having a portion extending from said emissive portion anddisposed inthe path of discharge, said'extended portion being electronemissive when heated by the discharge and so positioned with respect toits coated portion that the discharge is carried by said extendedportions to the substantial exclusion of said emissive material when thelamp is in operation.

' 16. A compound electrode having a non-metallic portionelectronemissive when subjected to agiven voltage when cold and held ona metallic mesh portion, and another metallic portion electron emissiveat a higher voltage when cold and so positioned with respect to saidfirst portion as to be heated during the operation of said firstportion, said high voltage portion being so positioned with respect tosaid first mentioned portion as to become electron emissive foroperation to the substantial exclusion of said first portion. as

JOHN W. MARDEN. GEORGE MEISTER.

